Patient-related research in heart problems in children has been of limited quality, because of lack of collaboration and emphasis on information provided in medical charts. Important problems remain controversial and unsolved. The Fontan operation is designed for patients who have only one pumping chamber, and guides blood from the major veins returning to the heart directly into the lung arteries, with the pumping chamber reserved to pump blood into the arteries of the body. Afterwards, blood flows differently and more slowly in the veins and lungs, and clots may form, which can cause strokes. The proposed study aims to determine the risk of clots and the best way to prevent them. The study will consist of a review medical charts on a large number of patients who have had Fontan to help determine the risk of clots. For patients who will have Fontan, they will be enrolled in a study where they will be randomly assigned to different types of medications to prevent clots, which will be compared. For patients who have already had the Fontan, they will be enrolled in a similar study where they will randomly assigned to either aspirin or a medication called warfarin. In both the studies, the patients will have regular check-ups and tests for a two year period to see if clots occur and which type of medicine best prevents the clots. Atrial septal defects, or holes between the two collecting chambers of the heart, can lead to heart failure and heart rhythmn problems. They can be closed with surgery, or by placing a special patch or device with a catheter. The proposed study aims to determine whether closure is better with surgery or the catheter method, and whether there are any differences in closure with different types of catheter devices. The study will review medical charts of children who have had closure of these holes, and compare surgery and catheter methods. A second part of the study will enroll children with holes which need to be closed, and randomly assign them to either surgery or the catheter method. For patients assigned to the catheter method, they will be further randomly assigned to one of two types of catheter devices. Comparisons will be made between surgery and the two catheter methods regarding the completeness of closure and complications. A further analysis will look at patient preferences and the costs for these different types of procedures, to help determine the best way to close these holes.